On the role of hydrogen-bond exchanges in the spectral diffusion of water

Abstract

The dynamics of a vibrational frequency in a condensed phase environment, i.e., the spectral diffusion, has attracted considerable interest over the last two decades. A significant impetus has been the development of two-dimensional infrared (2D-IR) photon-echo spectroscopy that represents a direct experimental probe of spectral diffusion, as measured by the frequency–frequency time correlation function (FFCF). In isotopically dilute water, which is perhaps the most thoroughly studied system, the standard interpretation of the longest timescale observed in the FFCF is that it is associated with hydrogen-bond exchange dynamics. Here, we investigate this connection by detailed analysis of both the spectral diffusion timescales and their associated activation energies. The latter are obtained from the recently developed fluctuation theory for the dynamics approach. The results show that the longest timescale of spectral diffusion obtained by the typical analysis used cannot be directly associated with hydrogen-bond exchanges. The hydrogen-bond exchange time does appear in the decay of the water FFCF, but only as an additional, small-amplitude (<3%) timescale. The dominant contribution to the long-time spectral diffusion dynamics is considerably shorter than the hydrogen-bond exchange time and exhibits a significantly smaller activation energy. It thus arises from hydrogen-bond rearrangements, which occur in between successful hydrogen-bond partner exchanges, and particularly from hydrogen bonds that transiently break before returning to the same acceptor.

Using Activation Energies to Elucidate Mechanisms of Water Dynamics

“…We have recently used this approach to examine many dynamical and equilibrium properties of water [16][17][18][19][20][21][22][23][24][25][26] (and other systems [27][28][29] ) including OH reorientation. [17][18][19][20][21] and self-diffusion.…”

“…Such plausible arguments about the connection between the spectral diffusion and jump timescales are directly tested by examination of the corresponding activation energies. 26 We have recently calculated the spectral diffusion and jump activation energies, E a,ω and E a,0 , respectively, for three water models. The results are presented in Fig.…”

“…4), which shows that H-bond exchange is not required for all water dynamical properties. 26 The fluctuation theory for dynamics approach that enables much of this analysis also permits non-Arrhenius parameters to be calculated from simulations at a single temperature, 16,21 as illustrated in Sec. 5.…”

“…The principal difference is that they decay on time scales faster than the jump time because they Such plausible arguments about the connection between the spectral diffusion and jump time scales are directly tested by examination of the corresponding activation energies. 27 We have recently calculated the spectral diffusion and jump activation energies, E a,ω and E a,0 , respectively, for three water models. The results are presented in Figure 5 and show that the spectral diffusion time has a significantly lower activation energy than the jump time.…”

Altered relaxation dynamics of excited state reactions by confinement in reverse micelles probed by ultrafast fluorescence up-conversion

“…34 Indeed related correlation functions are accessible from a range of ultrafast multidimensional nonlinear optical experiments (where C(t) is often described as a frequencyfrequency correlation function). [34][35][36][37][38] These interrelations are important as they connect the observables of numerous time domain experiments to the medium dynamics (for examples see Section 4) which can in-turn have a controlling influence on reactive dynamics (see Section 3) and which are accessible through molecular dynamics simulations. Such multiple means of characterising medium dynamics are important in understanding complex phenomena such as reactions in solution.…”